It is essential in numerous liquid-feed systems to provide means that prevents any reverse flow of the liquid in a particular line or conduit. Thus in a drinking-water supply system it is absolutely essential that the pure-water supply be protected from flow back into it of water from the mains or installations to which it is connected. It is also essential in chemical plants and the like to provide many liquid supply lines with similar such protective devices which, for instance, prevent a solution from being forced back into a line through which the solvent is fed.
Accordingly there is known a backflow preventer valve as described in my earlier U.S. Pat. No. 3,903,916. Such an arrangement is built into the conduit to be protected. It forms an open fluid connection so long as the pressure upstream of itself remains above a predetermined minimum. When, however, the pressure upstream of such a backflow preventer falls below the predetermined minimum this preventer not only closes but also opens up the downstream portion so as to depressurize same. Thus in the event that the supply pressure drops so that the pressure in the downstream portion could cause reverse flow, this arrangement will surely and safely segregate the two lines and drain the downstream portion.
Such an arrangement has been found extremely practical. Nonetheless in one particular circumstance it has been found to be inadequate: when the pressure downstream of the backflow preventer rises above the pressure upstream, with no dropping of the upstream pressure. Although a checkvalve is normally provided in such a backflow preventer to prevent any reverse flow in this situation, experience has shown that such a checkvalve alone is often insufficient to adequately protect the upstream fluid supply, as such a checkvalve operates so rarely that it frequently becomes wedged, and furthermore because the pressure downstream can occasionally build up to a level which the checkvalve cannot resist.